Whatever method you prefer, here’s a tip that may save you some grief down the road: When installing a new set of tubes, wait until night-time. Put ’em in, bias ’em up, and turn off the lights. Wait for 10 full minutes, until your eyes are fully adjusted to the dark. Then, observe the plates in the output tubes. You can sometimes find a hot-spot that indicates trouble down the road, or observe that the whole set is running hot; regardless of the specs, you might want to back the current off a little. Many times, these hot-spots just aren’t visible in normal room lighting, and especially not in the lighting many techs use on the bench. (I use a whole lot, I’m damn near blind.)

Arc-hunting…

Another thing to do in the dark: Chasing arcs is always a bummer… sometimes, you have one that’s caused by a wire broken inside the insulation, or one that’s between the fiberboards, or even one inside a cracked resistor; I even found a coupling cap that was arcing internally this way… Again, power up the amp and turn off the lights. I like to do this with a test load connected, so you can possibly HEAR the arc as well as see it. After your eyes adjust to the dark, beat the chassis with a rubber hammer. (I also occasionally excite the chassis with a finishing sander…after removing the sandpaper, of course.) If anything’s sparky, you’re gonna see it plain as day. Even arcs inside wire insulation and components emit enough light for you to plainly see them, once your eyes are sensitive enough. I once found an arcing standby switch in an old Marshall with this technique, after it had flummoxed damn near every tech in town. As for arcs between fiberboards, sometimes you can see the light that’s emitted outwards >from the crack; if not, get as close to edge-on as you can when you observe them.

By the way…be DAMN careful where you put your fingers when the lights are out. (Ouch!)